1. Field of the Invention
The present invention relates to a hologram device and a method for producing a hologram and, in particular, to a screen hologram where a hologram is used for a screen and a method for producing such a screen hologram.
2. Description of Related Art
A screen hologram is known in the prior art. See, for example, Japanese Examined Patent Publication No. 52-12568. In such a screen hologram, a content of a display device is imaged on a transparent screen created under a principle of a hologram, so that a viewer can see the image, while allowing the viewer to see the background via the transparent screen. When the display device is not in use, the screen hologram acts only as a transparent plate.
Such a screen hologram is used, for example, for displaying a content such as an advertisement on a show window in a department store or an underground shopping mall and for displaying a necessary information at counter for a customer or a patient in a bank or hospital, while allowing a receptionist to identify the customer or patient. In the unused state, the screen hologram merely functions as a transparent plate, thereby preventing a view from being hindered by the hologram screen.
FIG. 52 shows a schematic arrangement of a display device using a transparent type of a screen hologram. At a rear side of the screen hologram 1 at a location above a screen hologram 1, a device for projecting an image such as a projector 3 is arranged. Light 31 from the projector 3 is projected to the screen hologram 1, so that an image is created on the screen hologram 1. Light 32 is thus emitted from the screen hologram 1, so that a viewer on a front side of the hologram screen can see a displayed image on the screen hologram. Furthermore, a background light 33 passes through the hologram screen 1 without being hindered, which allows the viewer to see not only the displayed content but also the background 7.
A modified arrangement is possible, where the projector 3 is arranged at the back side of the screen hologram 1 at a location below the hologram 1.
As a further modified arrangement, in place of the transparent type of hologram 1, the display device can employ a reflection type of screen hologram, which is per se, known. In such a reflection type, the projector is arranged on the front side of the hologram, so that a viewer on the front side of the hologram can see the displayed content.
Now, a system for producing a transparent type screen hologram will be explained with reference to FIG. 53. A reference numeral 110 denotes a laser light generator, from which a laser beam is emitted and is, at a beam splitter, divided into two beams 112 and 113. The first divided beam 112 is directed to an object lens 122, so that a diverted light is obtained, which is directed to an off-center concave mirror 114, so that a parallel beam 115 is obtained. The parallel beam 115 is passed through a light diffusing body 116 constructed, for example, as frosted glass, so that a diffused light, as an object light 117, is obtained. On the other hand, the second divided beam 113 is introduced into an object lens 121 to obtain a diverted light as a reference light 118.
The object light 117 and the reference light 118 are directed to a photosensitive member 120, so that light interference occurrs, whereby generating a light interference fringe which is recorded on the photosensitive member 120. In this case, the light diffusing body 116 as a hologram is recorded in the photosensitive member 120. Thus, during regeneration process, the light diffusing body is re-generated, so that diffraction and diffusion of the re-generating light, at the hologram, occurrs. As a result, the hologram, from which the diffused light is emitted, functions as a screen.
FIG. 53 illustrates a method for the production of a screen hologram of a transparent type, where exposures to the object and reference lights are done on the same side of the photosensitive member 120. In case of a reflection type screen hologram, exposures to the object and reference lights are done on the opposite sides of the photosensitive member 120.
Furthermore, in the illustrated method, an arrangement of the optical system for an exposure is such that the laser beams, i.e., a reference light and an object light, are passed through the same horizontal plane, which allows the hologram of the desired size to be obtained by an execution of a single exposure process. This horizontal plane is, below, referred as an "exposure horizontal plane".
However, when a hologram produced by the method as explained with reference to FIG. 53 is used in an optical system for a regeneration as shown in FIG. 54, a projection of a white light to the hologram may cause, in a field of view of an observer, the upper part to have a blue color and the lower part to have a yellow or red color.
In order to overcome this problem, Japanese Examined Patent Publication No. 52-12568 proposes a multi-stage exposure method, where a series of exposures to a photo-sensitive member are done by using laser beams of red, green and blue color, respectively. As an alternative, exposures using laser beams of red, green and blue colors are done to separate photo sensitive members, which are, then, laminated.
However, these methods in the prior arts are defective in that three laser devices for laser beams of red, green and blue colors are necessary, which makes the system complicated.
Furthermore, the prior art method in FIG. 53 is also defective in that a desired quality of a hologram cannot be obtained when a large size of a hologram is needed due to a limitation in the size of the diffusing body. The reason will be explained with reference to FIG. 54. When a light from the projector 3 is projected to the screen hologram 1 for executing a re-generation of the screen hologram 1, the projected light is subjected to a diffraction at the screen hologram 1, so that a scattered light 11 having the same scattering property as that recorded in the hologram is obtained, which allows an observer to view an image on the screen hologram. In this case, a view range of the screen hologram is defined as an area in which an observer can view an image of a light diffusing body recorded in the photosensitive member. Thus, the size of a diffusing body recorded in the screen hologram 1 determines the view area of the screen hologram 1.
Now, a determination of the size of the diffusing body for producing a screen hologram having a desired view area will now be explained with reference to FIG. 55. In FIG. 55, a diffusing body is designated by a reference numeral 116. In front of the diffusing body 116, a photo-sensitive member 120, having top and bottom ends 120a and 120b, is arranged at a distance L1. A viewer 5 is located in front of the photo-sensitive member 120 at a distance L2. A view range is defined by a reference numeral 125 having ends 125a and 125b. The size of the diffusing body 116 is determined by the area of the diffusing body 116 defined by lines a1 and b1, where the line a1 is a line connecting the lower end 25b of the view area 125 with the upper end 120a of the photo-sensitive member 120 and a line b1 is a line connecting the upper end 125a of the view area 125 with the lower end 120b of the photo-sensitive member 120.
The size of the diffusing body 116 is actually limited by a particular arrangement of the optical system, that is determined by the value of the distance L1 between the photo-sensitive member 120 and the diffusing body 116 and the distance L2 between the photo-sensitive member 120 and the location of the viewer 5.
In short, the necessary size of the light diffusing body 116 for an exposure of the photo-sensitive element 120 is thus determined. Namely, the larger the size of the photo-sensitive member 120, the larger must be the size of the light diffusing body 116. In particular, in the case of an application of a screen hologram to a display device such as a show window, a hologram screen of an increased size is needed, which makes the size of the light diffusing body to be highly increased. For example, in order to produce a screen hologram of a diagonal size of 1 meter and of a view area (view angle) of .+-.20.degree., a diffusing body of a diagonal size of 3 meter is needed. A use of a diffusing body of such an increased size is actually impossible due to a limitation in a designing of an optical system. Namely, a limitation of the size of a light diffusing body makes it difficult to obtain a hologram screen of an increased size.
As another problem generated when producing a screen hologram of an increased size, it is necessary that, in FIG. 53, the light 115 for illuminating the light diffusing body 116 for producing the object light 117 must be expanded to a size larger than the size of the light diffusing body 116 and the reference light 118 must be expanded to a size larger than the size of the photo-sensitive member 120 for the production of the hologram. This makes the intensity of the laser beam to be reduced per unit area of the photo-sensitive member 120, which prolongs the exposure time, which is much longer than a period in which interference fringes stay in a stable condition on the photo-sensitive member 120, which makes the production of a hologram of an increased size very difficult. Namely, as is well known, an energy of a laser light for an exposure of a photo-sensitive member for a production is determined in accordance with a particular material constructing the photo-sensitive member. For example, a desired exposure amount for a photo-sensitive member is in a range between 100 to 300 mJ/cm.sup.2 in case of a dichromatic gelatin and between a few to 50 mJ/cm.sup.2 in case of a photo-polymer. Thus, an exposure time in a range of 30 minutes to a few hours is needed even when a laser of an output power of 10 watts is used. On the other hand, the time which is allowed for a production of a hologram is usually as short as about 10 minutes, which is much shorter than the above mentioned desired exposure time, which makes it very difficult to produce a hologram.
In short, the prior art is defective in that a production of a screen hologram of a increased size is difficult due to the requirements of an increased size of a light diffusing body and an prolonged duration of an exposure time.